The present invention relates to a system and method for determining the position and orientation of a remote object relative to a reference coordinate frame using magnetic fields. More specifically, the position and orientation of a medical device, such as a catheter, within a patient is determined.
Various magnetic tracking systems use circular loop antennas for magnetic field generation and detection. One or more coils generate distinguishable magnetic fields and one or more coils detect the generated magnetic fields. Knowing the position, orientation and geometry of the generating coils and the signal sensed in response to the magnetic field generation, the position and orientation of the sensing coils can be determined with respect to the field generators. The magnetic field generators usually establish the coordinate reference frame from which measured position and orientation is referenced. Depending on the number of coils used for generation and sensing, measurement of values for up to 6 degrees of freedom (three position values and three orientation values) of an object can be obtained. The equations normally used for calculating position and/or orientation using a generated magnetic field are almost all based on the assumption that the distance between the generating and detecting coils (r) is larger than the radius (a) of the generating coil. This equation is known as the infinitesimal dipole model. This assumption is explicitly used in the following U.S. patents, hereby incorporated by reference.
Hansen (U.S. Pat. No. 4,622,644) discloses a 5 degree-of-freedom (5DOF) position and orientation (P&O) measurement system that uses 2 or 3 sets of orthogonal Hall effect sensors and a bar magnet for generating a magnetic field. It is noted that using 2 sets of 3-axis orthogonal Hall effect sensors restricts the region of operation due to ambiguities. A noise error vector is noted that contains the estimated residual noise measured at the sensors. His method of solution starts by determining an approximate starting point for a calculation process that solves the system of 9 non-linear equations in 5 unknowns (x position, y position, z position, bar magnet azimuth, bar magnet elevation). The non-linear equations are based on the B-field approximation. Likewise, Hansen (U.S. Pat. No. 4,642,786) uses the same P&O solution approach in a variety of tracking system configurations (both 5DOF and 6DOF) using the retransmission of magnetic fields.
Rorden et al. (U.S. Pat. No. 4,710,708) discloses locating methods using various generating and detecting coils for underground surveying. These include 2, 3, or more, 3-axis orthogonal detecting (sensing) coil sets with a single transmitting coil; a 2-axis orthogonal generating coil set with a 3-axis orthogonal detecting set; a plurality of single axis collinear generating coils with a three axis orthogonal detecting set, among others. It is noted that at least 6 coil elements are required to avoid ambiguities or additional information must be utilized. Also noted is that using 2 sets of 3 axis orthogonal detecting sets restricts the region of operation due to ambiguities. The method of solution utilizes a process that solves the system of 6 non-linear equations in 6 unknowns (x position, y position, and z position, x direction cosine, y direction cosine, and z direction cosine). The direction cosines are composed of trigonometric combinations of azimuth and elevation such that this set of equations can be reduced to 5 unknowns. The non-linear equations are based on the B-field approximation.
Bladen et al. (WO 94/04938) discloses two system configurations: 3 3-axis orthogonal generating coil sets with a single axis detecting coil (5DOF) and one 3-axis orthogonal generating coil set with a 3-axis orthogonal detecting coil set (6DOF). Bladen's Appendix A describes the approximate B-field equations utilized in their process. Like Hansen, the 5DOF method of solution starts by determining an approximate starting point for the process and then solves a system of 9 non-linear equations in 5 unknowns (x position, y position, z position, azimuth and elevation).
Dumoulin et al. (U.S. Pat. No. 5,377,678) starts with equations that are remarkably similar to Rorden's equations (again, B-field approximations). He never mentions the magnetic moment (m) from Rordens equations and has a 5DOF system with 5 receiver coils and a single axis generating coil. He solves 5 non-linear equations in 5 unknowns (x position, y position, z position, azimuth and elevation) and notes that there must be at least 5 coil elements. To start the process he places the generator coil at a known position and orientation. As an aside, he first states that two transmitter coils are needed to determine both position and orientation (column 2, lines 66 on) and then states he needs one transmit coil to determine position and orientation (column 4, lines 41 on).
While it may not be obvious, a number of other patents use the infinitesimal dipole approximation, including Jones (U.S. Pat. Nos. 4,737,794, 5,307,072), Raab (U.S. Pat. Nos. 4,054,881, 4,314,251, 4,346,384), Crow et al. (U.S. Pat. No. 4,328,548), Egli et al. (U.S. Pat. Nos. 4,287,809, 4,394,831), Constant (U.S. Pat. No. 4,396,885), Blood (U.S. Pat. No. 4,613,866) and Kuipers (U.S. Pat. No. 4,742,356).
Published PCT application WO 97/32179 to Acker discloses a location system wherein at least one field generating coil or device creates different electromagnetic fields during different time intervals. This system specifically covers pulsed-DC field generation and excitation control. It also describes a non-linear equation solution to the tracking problem, which has already been covered by others.
A few patents note needed corrections when using the infinitesimal dipole model to make it more accurate when working close to the generating coil. Jones (U.S. Pat. No. 4,737,794) builds upon the dipole model and discloses a quasi-closed form (non-iterative) matrix solution of P&O using one 3-axis orthogonal generating coil set with a 3-axis orthogonal detecting coil set (6DOF). As noted above, Jones starts with the dipole approximation. He then introduces additional correction terms from an infinite series of Legendre polynomials with the unstated proviso that the distance between the generating and receiving coils (r) is greater that the transmitting coil radius (a). These correction terms are calculated based on the previous P&O solution. He casually mentions that elliptic integrals can also be used for generating these correction terms to the dipole model.
Jones (U.S. Pat. No. 5,307,072) discloses methods for compensating for positional offsets of generating coil and sensing coil sets. These methods are deeply embedded into his P&O process disclosed in 4,737,794. As noted above, Jones starts with the dipole approximation. He then introduces additional correction terms from an infinite series of Legendre polynomials, again with the unstated proviso that r&gt;a that correct for coil size, geometry and coil placement. These correction terms are calculated based on the previous P&O solution but can be used directly. The use of elliptic integrals, hypergeometric functions, etc. is also mentioned as a means of generating these correction terms to the dipole model. The methods detailed in both Jones patents will yield incorrect results when r&lt;a. Neither will work at all when r=0 (the sensor coil collocated within the generating coil).
Extending these concepts further, certain patents address generating coils of any shape. They also require the use of non-linear equation solving to provide part or all of their position and orientation solution. Ben-Haim et al. (WO 96/05768) discloses a position and orientation measurement system that uses any shape generating coils. He too uses Legendre polynomials to define the magnetic field structure, correct for r&gt;a only. He claims as new to the art non-concentric sensing coils for his 6DOF approach but Jones (U.S. Pat. No. 5,307,072) has already addressed this issue.
Blood (U.S. Pat. No. 5,600,330) discloses a system to determine P&O using 2 or 3 non-dipole generating antennas (e.g., rectangular coils) that are not necessarily orthogonal. It does use a 3 axis orthogonal detecting coil set and yields a 6DOF solution. The distortion immunity and large volume of measurement space are obtained by constraining the sensing device to work within the perimeters of the generating coils. It is noted that a process that iteratively determines P&O could have been developed. It is also noted that the number of receiving antenna (i) times the number of transmitting antenna (j) must equal at least 6 with the requirement that i and j must at least be two (for 6DOF). It is noted that the location, orientation and geometry of the coils must be known, implying that the B-field equations can be explicitly stated. A non-linear process solving 6 equations in 3 unknowns (x position, y position and z position) based on measured magnitudes and vector dot products is developed. An exact non-iterative process is then used to compute orientation. Bloods process eliminates the need to compute elliptic integrals or Legendre polynomials but only works well within the confines of the generating coils.
Other patents that use non-dipole fields in very different ways include Acker et al. (U.S. Pat. No. 5,558,091), Martinelli (U.S. Pat. No. 5,592,939) and Voisin (U.S. Pat. No. 5,172,056). These disclosures use multiple coils to form fields that are not dipole in nature but vary quasi-linearly in certain respects.